This invention relates to a method of partially plating a synthetic resin article.
A metal coating can be formed on an article made of a synthetic resin by electroless or chemical plating, though the metal must be selected among only a few kinds. For synthetic resin articles, it is frequently required that only a specific region (or regions) of each article be plated with a metal for an ornamental purpose, for example, but the requirement cannot easily be met in industrial production.
The most familiar technique for accomplishing partial plating of a synthetic resin article is a masking method, wherein areas not to be plated are coated with a certain material which prevents the deposition of a metal thereon by chemical plating. The masking material is usually a resin-base paint or ink. This method has the shortcoming that a significant discoloration of the antideposition coating occurs as the result of an oxidizing acid treatment of the article by an etching procedure preparatory to a chemical plating operation. Besides, the deposited metal is liable to exhibit a significant accumulation along the boundary between the plated coating and the anti-deposition coating and/or irregularly spread across the boundary. Accordingly the plated article suffers from insufficient precision in the geometry of the plated coating and/or unsatisfactory ornamental effect, and the cost of plating is relatively high due to a large number of unacceptable products.
As an alternative to a partial plating technique it is known to initially form a metal coating by chemical plating on the entire surface of the article and then cover the unwanted region of the plated coating with a nonmetallic paint. This method too has some disadvantages such as the waste of the metal material, relatively high cost, peeling tendency of the nonmetallic coating due to its insufficient adhesion to the plated coating, and a low wear resistance of the non-metallic coating.
In some cases, a local coating of a synthetic resin article with a metal layer is accomplished by a hot-stamping technique. However, this technique is not always applicable to variously shaped articles. Besides, a metal coating formed by this method is inferior in durability to a coating formed by chemical plating.
The appearance of a chemically plated coating will be improved by performing electroplating on this coating. In this case the article must be connected to the anode of an electroplating apparatus at a point on the chemically plated coating, and every region to be plated must be connected to this point. Accordingly this method is impracticable when the plating of either a plurality of discontinuous regions or only a middle region of a definite surface is required.
Synthetic resins can be rendered conductive by dispersing therein a finely divided conducting material such as carbon black. It seems possible to effect electroplating directly on a resin article by using a conductive resin as the material of the article. However, this idea has not been realized, at least industrially, since the values for the intrinsic specific resistance of currently available conducting resins are too large to accomplish electroplating on these resins. The specific resistance can be decreased by increasing the amount of the conducting material added to the basic resin. However, a conducting resin containing a large amount of a conducting material is quite unsatisfactory in its fluidity at shaping operations and, besides, loses toughness and/or elastomeric character in a shaped state. At present it is difficult to obtain a conducting resin which has a specific resistance small enough to the accomplishment of a direct electroplating thereon and nevertheless retains good physical properties required for a resin as a moldable structural material.